There are various metal reduction processes of producing reduced iron and alloy iron, and among the processes, the operation of a rotary hearth furnace (hereinafter, referred to as the RHF) is performed as a process having good productivity with low cost. For example, a brief outline of the process is described in Patent Document 1. FIG. 1 shows a cross-section in a diameter direction of the rotary hearth furnace. As shown in FIG. 1, the RHF is a baking furnace of a rotary type (hereinafter, referred to as the rotary furnace) in which, under a fixed refractory ceiling 1 and side walls 2, a disk-shaped refractory hearth 4 with a hollow center portion mounted on wheels 3 rotates on a rail 5 drawing a round circle at a uniform rate. The side walls 2 have a plurality of burners 6 installed therein. Fuel and air are injected from the burners to control an atmosphere gas component and a temperature in the furnace. Generally, a diameter of the hearth of the rotary furnace is in the range of 10 m to 50 m and a width thereof is in the range of 2 m to 8 m. A cast of powder including oxidized metal and carbon, corresponding to a raw material, is supplied onto the hearth 4 and heated by radiation heat from gas of an upper portion in the furnace. By the reaction of the oxidized metal and the carbon in the cast, metal is obtained in the cast.
FIG. 2 shows an example of the whole equipment of the RHF. For a raw material, oxidized metal such as an ore powder and oxidized metal dust and carbon acting as a reductant are used. In producing reduced iron, fine iron ore such as pellet feed or a by-product such as converter dust, sintered dust and blast furnace gas dust obtained from an iron-making process is used as an oxidized iron source. Coke, oil coke, coal or the like is used as the carbon acting as the reductant. It is preferable that the carbon acting as the reductant has a high carbon content (fixed carbon) that is not volatilized up to a temperature of about 1100° C. at which reduction reaction occurs. Such a carbon source is coke breeze or anthracite.
First, in a ball mill 11 which is a mixing device of FIG. 2, a powder including oxidized metal and a powder including carbon are mixed and then the mixture is molded to be granulated by a granulator 12. The resulting cast is supplied so as to be uniformly spread on the hearth 4 of a rotary furnace 13. In the rotary furnace 13, the cast moves through the portions in the furnace while the hearth 4 rotates. The cast is heated to 1000° C. to 1500° C. by hot gas radiation so that the carbon in the cast reduces the oxidized metal. Exhaust gas generated in the furnace passes through an exhaust gas duct 14 and is subjected to heat recovery by a boiler 15 and a heat exchanger 16. Then, after being subjected to dust removal by a dust collector 17, the gas is discharged to the air from a chimney 18. In the rotary furnace 13, the cast stands on the hearth 4 and thus there is an advantage that the cast is difficult to be broken in the furnace. As a result, there is a merit that a problem caused due to the adhesion of the powderized raw material to refractory does not occur. In addition, there is also an advantage that coal-based reductant and a powder raw material which are inexpensive and have high productivity can be used. A metallization ratio of reduced iron produced in this manner is 93% or less, and the reduced iron is slightly low in reduction degree as compared with direct-reduced iron (DRI: Directly Reduced Iron) produced by a gas reduction such as a MIDREX method.
For example, as described in Patent Document 2, there is also a method of producing high-strength reduced iron. The high-strength reduced iron is supplied together with lump ore or sintered ore to a blast furnace to produce pig iron. In this method, pre-reduced oxidized iron is finally reduced and molten in the blast furnace and thus heat load of the blast furnace is reduced. Accordingly, there are effects on reduction of coke source unit of the blast furnace and on increase of a pig iron producing amount.
Meanwhile, the DRI produced by the gas reduction such as the MIDREX method, which is a method of producing oxidized iron other than the RHF, has high porosity, and as a result, reoxidation of the metal iron easily occurs as a problem. In order to solve the problem, the DRI is hot-molded by a device shown in FIG. 3 as described in, for example, Patent Document 3 and Patent Document 4. In this molding method, a powdery or granular raw material largely including reduced iron is left to a relatively high temperature of 1000° C. or lower and the reduced iron supplied from a raw material chute 21 is sandwiched in between a pair of rollers 23 having recessed molds 22 to produce reduced iron casts 24 (hot briquette iron (HBI)). The reduced iron casts 24 are cooled up to a room temperature in a water-cooling device 25. In the hot briquette method, since the metal iron is pressed to be molded, it is preferable that a ratio of the metal iron in the DRI is high in order to produce suitable casts. Generally, DRI particularly having a high iron metallization ratio is molded and a ratio of metal iron in a raw material is in the range of 90% to 98%. When the ratio of metal iron is set as described above, a high-strength cast can be produced without a particular molding technique.
The HBI (reduced iron cast) has high density and is characterized in that inside thereof has few pores. Accordingly, the HBI is difficult to reoxidize and has high loading density and thus long-term storage or transport thereof can be performed. In addition, due to a dense structure thereof, there is an advantage that a melting rate in a melting furnace such as a steel-making electric furnace is high. Currently, hot briquette equipment is installed in many reduced iron plants. In usage thereof, the HBI is used as a reduced iron raw material in a vertical melting furnace or a steel-making electric furnace as in a method described in Patent Document 5.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2001-303115    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-218019    [Patent Document 3] U.S. Pat. No. 4,934,665    [Patent Document 4] U.S. Pat. No. 5,547,357    [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H11-117010